4.3 Insulation resistance
_______________
INTERNATIONAL TELECOMMUNICATION UNION
CCITT K.28
THE INTERNATIONAL
TELEGRAPH AND TELEPHONE
CONSULTATIVE COMMITTEE
PROTECTION AGAINST INTERFERENCE
CHARACTERISTICS OF SEMI-CONDUCTOR
ARRESTER ASSEMBLIES FOR THE
PROTECTION OF TELECOMMUNICATIONS
INSTALLATIONS
Recommendation K.28
Geneva, 1991
Printed in Switzerland
FOREWORD
The CCITT (the International Telegraph and Telephone Consultative Committee) is a permanent organ of the
International Telecommunication Union (ITU). CCITT is responsible for studying technical, operating and tariff
questions and issuing Recommendations on them with a view to standardizing telecommunications on a worldwide
basis.
The Plenary Assembly of CCITT which meets every four years, establishes the topics for
study and approves Recommendations prepared by its Study Groups. The approval of Recommenda-
tions by the members of CCITT between Plenary Assemblies is covered by the procedure laid down in
CCITT Resolution No. 2 (Melbourne, 1988).
Recommendation K.28 was prepared by Study Group V and was approved under the Resolution
No. 2 procedure on the 18 of March 1991.
___________________
CCITT NOTE
In this Recommendation, the expression "Administration" is used for conciseness to indicate both a telecom-
munication Administration and a recognized private operating agency.
aITU1991
All rights reserved. No part of this publication may be reproduced or utilized in any form or by any means, elec-
tronic or mechanical, including photocopying and microfilm, without permission in writing from the ITU.
PAGE BLANCHE
Recommendation K.28
Recommendation K.28
CHARACTERISTICS OF SEMI-CONDUCTOR ARRESTER ASSEMBLIES
FOR THE PROTECTION OF TELECOMMUNICATIONS INSTALLATIONS
Foreword
Careful survey of the electrical environment that telephone equipment must safely survive,
has led to the conclusion that semi-conductor devices that are robust enough to act as primary
protectors are now possible. Semi-conductor devices provide for tightly toleranced and stable
over-voltage control, which does not change with age or activity within their design capability.
Furthermore, they introduce negligible circuit noise on the circuits they are protecting.
Widespread trials of these semi-conductor overvoltage protectors for primary protection
are taking place and this Recommendation provides detailed guidance on the particular qualities
which should be sought when manufacturing or purchasing such devices. The trials and initial
applications are ongoing and some details of the technology may change in the light of the results.
Nevertheless, to bring the trials and initial applications to the notice of a wider audience and to
acquaint potential users with both the advantages and disadvantages of these devices, CCITT con-
siders the subject to be important and stable enough to publish a Recommendation on the new tech-
nology.
0 Introduction
The purpose of this Recommendation is to provide technical guidelines for purchasers and man-
ufacturers of semi-conductor arrester assemblies (SAA) to ensure their satisfactory operation
in the applications for which they are intended. Figure I-1/K.28 shows examples of such arresters.
It is intended to be used for the harmonization of specifications issued by manufacturers of
semi-conductor arrester assemblies (SAA) and network operators.
Only minimum requirements are specified for essential characteristics. As some users may
be exposed to different environments or have different operating conditions, service objectives or
economic constraints, the requirements of this Recommendation may be modified or further require-
ments added to suit local conditions. It is for Administrations to classify the environment for a
particular device, taking into account business policy, and economic and technical considerations.
The requirements detailed for arresters in this Recommendation may entail statistical
analysis of samples. Standard statistical analysis techniques can be applied and therefore no
description of this analysis approach is given.
1 Scope
This Recommendation applies to semi-conductor arrester assemblies to be used for primary
protection against voltage surges due to lightning or power disturbances on telecommunications
circuits, in accordance with Recommendation K.11. It deals with semi-conductor arrester assemblies
of the type that limit voltages from line to earth to a few volts when conducting sufficient current
to switch the device.
It does not deal with:
– mountings for SAAs and their effect on arrester characteristics;
– semi-conductor arresters which are connected in series with voltage-dependent resistors to limit fol-
low-on currents in electrical power systems;
– mechanical dimensions;
– quality assurance requirements;
– units containing heat-coils.
2 Definitions
These are given in Annex A.
3 Environmental requirements
The semi-conductor arrester assemblies should operate satisfactorily in, and be capable of
storage in, temperature and humidity ranges selected for the intended application. The selected tem-
perature range should be between the extreme values of –40 °C and +65 °C. The selected humidity range
should be between the extreme values of 0 and 95% RH.
4 Electrical requirements
4.1 Maximum voltage limiting
When tested according to _ 5.1, the SAA voltage limiting should not be outside the limits given in
Table1/K.28.
4.2 Minimum voltage limiting
The current in an SAA when tested according to _ 5.2 should not exceed the values given in Table
2/K.28 for the voltage limits shown.
4.3.1 This test measures the effect of two parameters simultaneously, semi-conductor junction
leakage and insulation resistance.
4.3.2 When tested according to _ 5.3, the values of combined leakage and insulation resistance
should not be outside the values given in Table3/K.28.
4.4 Capacitance
The capacitance between each pair of electrodes (excluding protector assembly capacitance)
should not exceed 200 picofarads (pF) when tested according to _ 5.4 at a frequency of 1MHz.
4.5 Impulse reset
The SAA should revert to its high impedance state in less than 30ms when tested to _ 5.5 using
appropriate rows(s) from Table4/K.28 of this Recommendation for the parameters that apply to
Figure1/K.28. Select the appropriate row depending on the expected SAA application.
Figure 1/K.28 = 11 cm
4.6 Rate of change of current
The SAA, when tested to _ 5.6, should not fail short circuit and should meet the maximum voltage
limiting requirement of _ 4.1 following application of the surge.
4.7 Surge life tests
4.7.1 SAAs should be measured for their performance in the categories of impulse life and 50/60 Hz
current carrying capacity, according to _ 5.7. The types of protectors in which the SAAs are
mounted for testing should be identified and the life tests should apply only when the SAAs are
used in those, or similar, protectors. Table5/K.28 of this Recommendation indicates the requirements.
The 10A impulse tests may be curtailed or waived if it can be demonstrated satisfactorily that no
wear-out mechanisms are present in the design.
4.7.2 The surge life requirements of Table5/K.28 may not be sufficient for protectors intended for
applications where they are directly connected to open wire lines, or in other high-exposure areas.
4.7.3 Devices that can be shown to have a surge life that is temperature sensitive should be
tested to _ 5.7 at the maximum and minimum operating temperatures for the intended application.
5 Test methods
5.1 Maximum voltage limiting (see _ 4.1)
5.1.1 The test current should be chosen from the range 10A to 100A. The maximum rate of change of
current applied to the SAA throughout the test shall not exceed 30A/ms. The device shall be tested with
both positive and negative waveforms.
5.1.2 With rate of rise as specified in Table 1/K.28, apply sufficient impulse voltage to cause breakdown. Repeat
the test with opposite polarity and using the same device. Allow a one or two second waiting time between applica-
tions.
5.1.3 For impulse testing, the voltage generator used for this test must be capable of maintaining the open circuit
voltage rate of rise of Table 1/K.28 (rate of rise defined in IEC 60).
5.2 Minimum voltage limiting (see _ 4.2 and Figure 2/K.28)
The generator shown in Figure 2/K.28 should provide a ramp of 100V/s to 100V/ms to the terminals
under test. The circuit current can be determined by monitoring the voltage drop across a 1kW resis-
tor. The generator voltage should be no more than the value shown in Table2/K.28.
Figure 2/K.28 = 7,5 cm
5.3 Insulation resistance (see _ 4.3)
Combined insulation resistance and leakage (known jointly as RI) should be measured between
each terminal and every other terminal of the SAA by applying a specified direct current voltage
source of both polarities with values as shown in Table3/K.28 of this Recommendation. Insulation
resistance readings should be taken after insulation stabilization or after one minute of applied
voltage, whichever occurs first. Terminals not involved in the measurement should be left float-
ing.
5.4 Capacitance (see _ 4.4)
The capacitance of the SAA shall be measured between each terminal and every other terminal.
All terminals not involved in the test shall be connected to an earth plane in the measuring instru-
ment. The measurement voltage should be small enough to not interfere with the measurement and
should in any case not exceed 1 V rms.
5.5 Impulse reset (see _ 4.5)
The maximum impulse current should be 25A, with a 10/1000 or 10/700 waveform measured through
a short circuit. The maximum rate of change of current applied to the SAA throughout the test
should not exceed 30A/ms. The impulse current should be applied to the SAA in the same polarity as the d.c.
source. Three impulses should be applied at not greater than one minute intervals. The tests should be repeated with
the specimen connections reversed. The 30ms requirement applies to the time between application of the impulse
and device reset. If required, the impulse generator may be disconnected from the SAA 10ms after the application of
the surge.
5.6 Rate of change of current (see _ 4.6 and Figure 3/K.28)
A surge with a rate of change of current of 25A/ms to 30A/ms and having a maximum current of 100A
and open circuit voltage of 1kV, should be applied to the SAA (see Figure 3/K.28). This should be repeated with
surges of the opposite polarity.
Figure 3/K.28 = 11,5 cm
5.7 Surge life tests (see _ 4.7 and Figure 4/K.28)
5.7.1 SAAs should be tested for impulse and 50/60 Hz life. When subjected to the various impulse and
50/60 Hz test currents shown in Table5/K.28, at 20 °C+2 °C, a sample should have a surge life in
accordance with the number of operations specified in that table. Half the specified number of
tests should be carried out with one polarity followed by half with the other polarity. Alterna-
tively, half the number in a sample quantity may be tested with one polarity and the other half
with the opposite polarity. Tests for failure by insulation resistance, maximum and minimum volt-
age limiting should be conducted after each application of the test currents. Impulse reset should
be measured after the number of operations specified as surge life criteria in Table5/K.28 of this
Recommendation for those SAAs surviving to that point.
5.7.2 The open circuit voltage for the impulse life tests should measure at least 1000 V peak. The
current amplitudes should be measured with the SAA replaced by a short circuit having minimum
inductance.
5.7.3 The test circuit for the 10 A rms alternating current test should consist of a 50/60 Hz sup-
ply feeding a parallel pair of non-inductive series limiting resistors, one for each line terminal.
The supply-resistor combination should deliver 1000 V rms under open circuit conditions and 10A
rms to each line terminal under short circuit conditions.
5.7.4 The 1 A alternating current tests should be conducted using the circuits shown in Figures
4a)/K.28 and4b)/K.28.
Figure 4/K.28 = 15,5 cm
5.7.5 A device should be considered to have reached end-of-surge life if any of the following
conditions apply:
1) the minimum voltage limit test results fall outside the limits in Table2/K.28;
2) the maximum voltage limit test results fall outside the limits in Table1/K.28;
3) the SAA fails to extinguish in less than 30ms at the component combinations listed for SAAs in
Table4/K.28;
4) the life test insulation resistance (RI) is less than or equal to 50 MW at 100V d.c.
6 Mechanical requirements
6.1 Mechanical durability
The SAA should be sufficiently mechanically durable to withstand normal installation and
maintenance procedures, as well as shipping, storage, and environmental stress.
7 High temperature conditioning
Samples should be subjected to the high temperature test specified here, and should display no
warping, fading, or degradation of any material within 12hours of return to ambient temperature.
They should be conditioned for seven days in a circulating air oven, maintained at the maximum tem-
perature of intended application with no humidity control. After the seventh day, the samples
should be removed from the oven and allowed to return to ambient temperature.
8 General test requirements
This section describes the performance criteria against which an SAA is analysed.
1) Certain tests require previous testing of samples for stress and environment. Also, subsequent tests
may be necessary to determine whether the samples are still operational. If possible, the former tests should be
completed, and these samples should proceed to the test programme with the untested samples.
2) Surge testing can cause semi-conductor devices to heat. Accordingly, allow sufficient cooling time
between surges, as recommended by the manufacturer.
3) In all testing, the rate of change of discharge current must not exceed 30A/ms at any time nor must the
specified peak current be exceeded. Monitoring equipment to record these parameters is recommended.
9 Product identification
9.1 Operating voltage identification
Each SAA should be marked in a clear, permanent, and distinctive manner to indicate the nominal
operating voltage.
9.2 Manufacturer's identification
On each SAA, the manufacturer's name, part number, and date code should be indelibly marked.
9.3 Customer's identification
If requested and agreed, the customer's identification should be indelibly marked, on each SAA.
10 Documentation
10.1 Complete instructions on installation and use should be included within every package of
SAAs (or should be available on request).
10.2 Instructions and documentation should indicate whether the enclosed devices should be
installed only in subscriber premises, or switching centres, or both.
10.3 Documentation should be provided so that the purchaser can determine the full character-
istics as set out in this Recommendation.
11 Ordering information
The following information should be provided by the purchaser:
a) a drawing giving all dimensions, finishes and termination details of protector package into which the
SAA will be fitted;
b) nominal limiting voltage;
c) the required markings;
d) quality assurance requirements.
ANNEX A
(to Recommendation K.28)
Definitions of terms special to this Recommendation
A.1 semi-conductor arrester (SA)
A semi-conductor device that is intended to go low impedance when the voltage across two termi-
nals exceeds a defined value, and go high impedance when that voltage is removed.
A.2 SA assembly (SAA)
One or more SAs assembled into a housing in such a way as to form a readily identifiable, pur-
chasable and testable unit. The function of an SAA is to divert overvoltages to earth, when
installed in a protector. Examples of SAA are shown in Figure I-1/K.28.
APPENDIX I
(to Recommendation K.28)
Figure I-1/K.28 = 16 cm